This invention relates to a method and apparatus for producing a holographic mirror by exposing a photosensitive layer to at least two interfering beams of coherent radiation so as to produce an interference structure in the interior of the photosensitive layer and parallel to its surface. As is known in the art, the interference structure is a standing wave, which in visible light can be seen as an array of fringes spaced at a distance that is a function of the wavelength of the interfering beams. The fringes are the loci of points in the two interfering beams having equal phase difference.
After exposure of the photosensitive layer to the interfering beams, the layer is developed in a known way to form a relatively permanent record of the standing wave which functions as a holographic mirror. Photosensitive layers of dichromated gelatin have proven particularly satisfactory for making such holographic mirrors.
In known processes of making holographic mirrors, a photosensitive layer is placed on a flat, transparent substrate, parallel to a mirror. It is exposed over its entire surface to a suitable laser beam that extends in two dimensions to cover such surface. The laser beam passes through the photosensitive layer and the substrate a first time and is reflected by the mirror so that it again passes through the photosensitive layer where it interferes with itself to form the desired standing wave. It is customary in the art to refer to the first laser beam as the reference beam and the reflected beam as the object beam.
This known process presents many drawbacks when making homogeneous holographic mirrors. For example, when producing extended holograms, it is extraordinarily difficult to keep the distance between the photosensitive layer and the surface of the mirror constant within the required precision throughout the exposure period. If it is desired to make a hologram having uniform reflective properties, this distance must be kept uniform over the entire surface with a precision on the order of fractions the wavelength of the illuminating laser beam because minute variations in this distance during the exposure time lead to changes in the position of the standing wave in the photosensitive layer and consequently obliteration or fading of the record that is made of the standing wave. Further, the maximum possible size of the hologram thus obtained is limited by the fact that it is difficult to achieve homogeneous illumination over large areas. Illumination can be performed segment by segment by exposing individual surface elements successively, but in this case it is inevitable that seams will become visible and bothersome.
Some fields of use require relatively extended, homogeneous holographic mirrors. For example, an automobile windshield may be equipped with such a holographic mirror, either on its entire surface or on a part of it, as descried in DE-OS 31 36 946. In this case, the hologram must be formed to reflect white light such as solar radiation from above while, in the horizontal direction, the light rays must pass through unimpeded. In addition, such holographic mirrors have also been formed and used in such a way as to make signals or optical information visible in the field of vision of the driver as described in EP 216 692.